In-creating-Customer-Indicator-Error-method-GetHistory-does-not-exist-in-the-current-context

In creating Customer Indicator - Why compiler says - Error - name GetHistory does not exist in the current context”

CODE:
using WealthLab.Core;
using System;
using System.Drawing;
using WealthLab.Indicators;
using WealthLab.Backtest;
using WealthLab.Data;
using System.Collections.Generic;
...
        public override void Populate()
        {
         BarHistory source = Parameters[0].AsBarHistory;
         BarHistory qqqBars = GetHistory(source, "qqq");
                        ...
       }

Thanks